Symmetrization of the Crystal Lattice of MAPbI3 Boosts the Performance and Stability of Metal–Perovskite Photodiodes

Semiconducting lead triiodide perovskites (APbI₃) have shown remarkable performance in applications including photovoltaics and electroluminescence. Despite many theoretical possibilities for A⁺ in APbI₃, the current experimental knowledge is largely limited to two of these materials: methylammonium (MA⁺) and formamidinium (FA⁺) lead triiodides, neither of which adopts the ideal, cubic perovskite structure at room temperature. Here, a volume-based criterion is proposed for cubic APbI₃ to be stable, and two perovskite materials MA_1−xEA_xPbI₃ (MEPI, EA⁺ = ethylammonium) and MA_1−yDMA_yPbI₃ (MDPI, DMA⁺ = dimethylammonium) are introduced. Powder and single-crystal X-ray diffraction (XRD) results reveal that MEPI and MDPI are solid solutions possessing the cubic perovskite structure, and the EA⁺ and DMA⁺ cations play similar roles in the symmetrization of the crystal lattice of MAPbI₃. Single crystals of MEPI and MDPI are grown and made into plates of a range of thicknesses, and then into metal–perovskite photodiodes. These devices exhibit tripled diffusion lengths and about tenfold enhancement in stability against moisture, both relative to the current benchmark MAPbI₃. In this study, the systematic approach to materials design and device fabrication greatly expands the candidate pool of perovskite semiconductors, and paves the way for high-performance, single-crystal perovskite devices including solar cells and light emitters.

Guided by a volume-based criterion, the tetragonal crystal lattice of MAPbI₃ is converted into an ideally cubic structure by introducing ethyl- or dimethylammonium, forming two mixed-cation perovskites MEPI and MDPI. Metal–perovskite photodiodes made of MEPI and MDPI single crystals show three times charge carrier diffusion length and ten times stability against moisture, both relative to the MAPbI₃ benchmark.

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